The invention relates to epoxy resin compositions, particularly, to an epoxy resin composition that when cured exhibits properties useful in structural assembly and, even more particularly, to two-part epoxy adhesive compositions that exhibit one or more improved adhesive properties such as impact, creep and fatigue resistance, as well as durability under service conditions for structural applications.
Adhesives have been used in many structural applications. Such structural applications have included vehicles, computer cases, buildings, appliances, etc. For example, structural adhesives have been used in vehicle assembly (e.g., automobile and aircraft assembly) to replace or augment conventional joining techniques such as welds, nuts and bolts, and rivets.
Epoxy compositions are known and have been used for structural adhesive applications. In state-of-the-art epoxy technology today, polymerization catalysts used to achieve higher order oligomers typically are tetraalkyl ammonium or phosphonium salts that do not promote epoxy homopolymerization. Cyclic amidine catalysts, such as imidazoline catalysts and imidazole catalysts, have also been used in adhesives. The adhesives of the prior art are quality adhesives in many applications. Even so, there is a continuing need for higher performance adhesives in order to meet the changing needs of various industries such as, for example, the vehicle assembly industry.
The present invention is intended, at least in part, to address the ongoing need for higher performance adhesives to meet the needs of various industries, including the vehicle assembly industry (e.g., automobile, aircraft and watercraft industry). Compositions of the invention are believed to be useful in structural adhesive applications either alone or in conjunction with conventional assembly techniques like welding and/or mechanical fastening (e.g., rivets).
We have found a composition useful as a structural adhesive having long term durability under static and/or dynamic loads and substantially improved impact, creep and/or fatigue resistance for use in assembly applications. The composition can include a chain extender, a catalyst, a reactive epoxy resin and one or more polymeric tougheners. At least when mixed together, the present adhesive composition is in a form that can be applied or dispensed (e.g., liquid or paste form). The chain extender, the reactive epoxy resin, the catalyst and the toughener are each in an amount and of a type that are effective, when mixed together, to form a thermally curable adhesive. When the adhesive is cured, at least about 50% by weight of the epoxy resin is chain extended. Preferably, at least 60 wt %, and even more preferably at least 70 wt %, of the epoxy resin is chain extended.
It is preferred that the composition be free, or at least substantially free, of a polyfunctional curing agent (i.e., an agent that chain extends and cross links the epoxy resin). That is, the amount of polyfunctional curing agent should be limited to the point that, when the adhesive is cured, the desired amount of the epoxy resin is chain extended.
The chain extender can comprise an amine, a phenolic compound or a combination thereof. Preferably, the chain extender is all or at least substantially in monomeric form (i.e., the chain extender is not prereacted, prepolymerized or in oligomeric form) prior to being added to the composition. That is, enough of the chain extender is in monomeric form to enable the resulting composition to be applied or dispensed. Preferably, the resulting composition is compatible with state-of-the-art dispensing and rheology (e.g., viscosity) requirements. It is also preferable that the chain extender be dissolvable into the epoxy resin, the catalyst or both, at least at an elevated temperature (e.g., the curing temperature of the composition). It may be desirable for the chain extender to be in solid particulate form and finely dispersed in the epoxy resin and/or the catalyst, where the chain extender dissolves at elevated temperatures.
The phenolic compound preferably includes a dihydric phenol (e.g., a di-hydroxy benzene, such as catechol, resorcinol and/or compounds based thereon), and the amine preferably includes a primary monoamine (e.g., attached to a primary or secondary carbon), a secondary diamine, and compounds based thereon. A polyfunctional or multifunctional amine (e.g., a diamine containing both primary and secondary functionality or multiple primary functionality) will cause chain extending and cross linking (i.e., will function as a curing agent). Even though it will cause cross linking to occur, a polyfunctional amine or other curing agent may be used, but in a limited amount.
The present composition can be a two-part adhesive with the catalyst in a part A and the reactive epoxy resin in a part B. The chain extender is included in at least one of the two parts. When the chain extender of such a two-part adhesive composition includes an amine, the amine is preferably only in the part A. It may be possible to add very small amounts of amine in the epoxy part B, as long as the amount of amine is not enough to adversely affect the part B (e.g., its rheology). When the chain extender of such a two-part composition includes a catechol, the catechol can be in the part A, in the part B or in both. It is preferable that the catechol is in at least the part A. It is surprising that the catechol can be sufficiently stable (i.e., not recrystalize or react) in the epoxy resin to provide a part B with a commercially acceptable shelf life. When the chain extender includes a catechol and resorcinol, at least the part A includes the resorcinol and catechol. The part B can include the catechol without resorcinol. When the chain extender includes another type of phenolic compound, it may also be included in the part A, part B or both.
It can be preferable for at least about 50 wt % of the chain extender to be catechol. When such a chain extender also includes resorcinol, up to about 50 wt % of the chain extender can be resorcinol. It is believed that the adhesive composition can contain in the range of from about 8 wt % to about 30 wt % of such a catechol and resorcinol containing chain extender, based on the amount of chain extender and reactive epoxy.
The catalyst is preferably a base. The catalyst can include a cyclic amidine, a tertiary amine, and substituted analogues thereof The catalyst can comprise one or more of imidazole, imidazoline, a substituted imidazole compound, a substituted imidazoline compound, 1,4,5,6-tetrahydropyrimidine, a substituted 1,4,5,6-tetrahydropyrimidine compound and combinations thereof The chain extender preferably includes catechol. The catalyst can also include one or more substituted pyridines, pyrrolidines and piperidines. The chosen catalyst or catalysts should not contain a group that exhibits an electron withdrawing or stereo chemical effect sufficient to prevent the composition, when mixed together, from forming a thermally curable adhesive suitable for structural bonding. Typically, as the mass of the catalyst increases, the amount of catalyst needed to establish a desired effect also increases, unless any substitution chemistry present has an affect on (i.e., increases or decreases) the effectiveness of the catalyst. The catalyst can comprise two or more different catalysts. We have surprisingly found that a combination of two different amidine catalyst species, in particular cyclic amidine catalysts, can provide enhanced adhesive properties. A preferred combination can include one or more imidazole compounds (substituted or unsubstituted) and one or more imidazoline compounds (substituted or unsubstituted). It is believed that a combination of a 1,4,5,6-tetrahydropyrimidine compound (substituted or unsubstituted) with an imidazoline compound and/or an imidazole compound may also provide enhance adhesive properties.
Preferably, the amount of the catalyst in the adhesive composition is at a level of at least about 0.5 wt-%, more preferably, in the range of from about 0.5 wt-% to about 10 wt-% or, even more preferably, in the range of from about 0.5 wt-% to about 7.5 wt-%, based on the total amount of the reactive species or components of the adhesive mass (i.e., the chain extender, epoxy resin and catalyst) and the molecular weight of the catalyst.
The reactive epoxy resin can comprise one or more glycidyl ether epoxy compounds, each having more than one reactive epoxy group. Preferably, the reactive epoxy resin comprises at least one of an aromatic glycidyl ether epoxy compound and an aliphatic glycidyl ether epoxy compound, with at least one compound having more than one reactive epoxy group. Typically, the reactive epoxy resin materials are present in amounts in the range of from about 50 wt-% to about 90 wt-%, and preferably about 80 wt-%, based on the reactive species of the composition (i.e., catalyst, chain extender and epoxy).
It is desirable for the adhesive composition to contain up to 35 parts, preferably in the range of from about 5 parts to about 35 parts, and more preferably from about 10 parts to about 30 parts, by weight of polymeric toughener based on 100 parts by weight of the reactive epoxy resin. For a two-part adhesive composition of the present invention, the toughener can be added to the part A, the part B or both.
The present adhesives may be used to supplement or completely eliminate a weld or mechanical fastener by applying an adhesive mass between two parts to be joined and curing the adhesive to form a bonded joint. Optionally, spot welding can be used to pin the parts together until the adhesive is sufficiently cured for handling. Welding can contribute to the curing process. The adhesives may be used to form assembled structures by applying the adhesives to augment or replace welded joints and other mechanical joints. Replacing or supplementing welded joints with an adhesive bond to create a load bearing joint is believed to require superior adhesive toughness over a broad temperature range in some applications, as well as adequate adhesion to the substrates being bonded. This is related directly to the degree of polymeric matrix ductility, which requires chain extension allowing for optimum toughening. Compatibility with state-of-the-art dispensing and rheology (e.g., viscosity) requirements for a flowable one- or two-part adhesive composition can require this chain extension to occur, at least substantially if not completely, after the adhesive is applied.
For the purposes of this patent application, the term xe2x80x9cactive hydrogenxe2x80x9d denotes a hydrogen atom in a chemical group wherein the group becomes chemically reactive with the oxirane group resulting in ring opening bonding to the group. Active hydrogens typically are found in amines, thiols, carboxylic acids and phenolics. Preferred active hydrogen groups include amine (xe2x80x94NHxe2x80x94, xe2x80x94NH2) groups and aromatic hydroxyl (xe2x80x94OH) groups. The function of the active hydrogen compound is to provide chain extension. Some cross-linking can be introduced by polyfunctional amines but only to a limited extent. If excessive cross-linking occurs the adhesive can lose toughness and adhesion. Conversely excessive chain extension with little epoxy homopolymerization will result in a weak adhesive.
The first part or part A of the two-part epoxy adhesive comprises the catalyst. The second part or part B comprises the reactive epoxy portion and optional toughener. One formulation places dihydroxy phenolic in the epoxy part B with only the catalyst in a part A. A second formulation places the amine and/or dihydric phenol along with the catalyst in the part A and the epoxy and a toughener in the part B. A third formulation places a portion of the phenolic in both the part A (catalyst) and part B (epoxy).
In the adhesive of the invention, we have found that the stoichiometric equivalents ratio of reactive hydrogen sites to reactive epoxy sites is preferably less than 1.0 (i.e., for each epoxy equivalent in the adhesive, there is less than 1.0 equivalents of active hydrogen). We have also found that it can be preferable for the stoichiometric equivalents ratio to be in the range of from about 0.5 to less than 1.0, in the range of from about 0.6 to less than 1.0, or in the range of from about 0.7 to less than 1.0. The active hydrogen sites can be provided by the chain extender and catalyst. Fillers or the toughener can be independently incorporated in either or both parts A or B. We have found that an amine of the type described above can replace a portion of the dihydric phenol without a loss in physical properties and may be useful as the only chain extender in the composition. The amine can act as a diluent for the Part A, to lower its viscosity, but may also shorten the work-life of the mixed adhesive. Another function of the amine is to reduce any tendency of the phenolic to recrystallize and help stabilize the viscosity of the Part A. A further function is providing latitude in formulating for a specific volumetric mix ratio to meet dispensing requirements.
Adhesives made using the formulations of this invention can obtain an impact peel strength of at least about 3 Joules, preferably at least about 5 Joules and most preferably at least about 10 Joules at a temperature in the range of from about xe2x88x9240xc2x0 C. to about 90xc2x0 C. The desirable impact peel strength depends, at least in part, on the type of substrates being adhered together. Further, adhesives according to the present invention can form adhesive bonds having a T-peel strength of greater than about 70 N/cm width at 23xc2x0 C., greater than about 85 N/cm width at 23xc2x0 C., and greater than about 100 N/cm width at 23xc2x0 C. The adhesive can sustain a load under certain accelerated environmental cycling conditions for at least 10 days, preferably greater than 20 days, most preferably greater than 30 days.
In another aspect of the present invention, a structure is provided that has a first surface and a second surface joined by an adhesive bond made with a cured mass of the above described adhesive composition. The structural adhesives of the invention can form high quality adhesive bonds between metallic components (e.g., iron, aluminum, titanium, magnesium, copper, etc. and alloys thereof), between non-metallic substrates (e.g., reinforced and unreinforced thermoplastic and thermoset polymers, as well as other organic materials or organic composite materials) and between metallic and non-metallic substrates. The structure being bonded can form at least a portion of a vehicle.
We have also found that adhesives used to augment or replace weld construction can provide useful properties to an assembly. Welded joints, while strong, tend to concentrate stress at the weld nugget perimeter and can fail at the weld perimeter if sufficient impact energy is applied to the joint. Additionally, corrosion resistance associated with the weld nugget and adjacent metal is typically reduced. Cured epoxy adhesives of the present invention can absorb substantial impact energy and dissipate the energy throughout the structure. Such energy dissipation properties, in conjunction with weld joints, can improve the survivability of a structure under conditions of high impact loads. Such an adhesive requires significant structural properties. Regardless of the direction of the impact energy, it may be desirable for the adhesive mass to be able to maintain structural integrity regardless of whether the adhesive is exposed to stress in a cleavage mode, a shear mode, a compression mode or a tensile mode. Therefore, the structure can comprise a joint having a welded bond in addition to the adhesive bond. In addition, the welded bond can be formed through the adhesive bond.
We have found two characteristics of adhesives that can help identify an adhesive that is useful in this type of application. We have found that the impact peel strength and T-peel adhesion of the adhesive can be useful indicators for adhesive utility. Other characteristics of adhesives useful as performance indicators can also include sustained load durability and fatigue resistance. The epoxy adhesives of the invention may be used in a structure having structural integrity that is maintained with both welded joints and adhesive bonds made using the curable adhesive of the invention or with only such adhesive bonds. Adhesives are also desirable, for example in the automotive industry, because in an effort to reduce weight, car manufacturers are looking to use thinner gauge steel either alone or in combination with aluminum, magnesium, etc. In addition, the present adhesives can be a viable option for bonding together various organic materials or composites which cannot be welded or joined with conventional methods. Additional benefits of a structure bonded according to the invention are believed to include improved crash worthiness (i.e., impact resistance), survivability, corrosion resistance, sealing of the joint and vibration damping.
In an additional aspect of the present invention, a method is provided for assembling the above described adhesively bonded structure. The method comprises the steps of: (a) applying an uncured mass of the composition of claim 1 to at least one of a first member and a second member; (b) sandwiching the uncured mass between the first member and the second member; and (c) curing the composition to form an adhesive bond so as to adhere the first member and the second member together. The first member can be a frame member and the second member can be a sheet-like member or another frame member. The method can also include the step of welding the sheet-like member to the frame member through the uncured mass before the curing step.
Adhesives can be used for such applications, but known structural adhesives that are currently available for such applications do not have the requisite combination of properties and performance over typical end use (e.g. service) temperature ranges. Such properties include long term durability and fatigue resistance under static and dynamic (e.g. cyclic) loads and good impact resistance.